The present invention relates to electro-optical imaging systems and, more particularly, to preamplifiers for solid state imaging systems.
Electro-optical imaging sensors are roughly divided into camera-tubes contained within evacuated envelopes and solid state imaging sensors in which a charge pattern is created by the impingement of light on a solid state matrix array. One type of solid state imaging sensor, which forms the environment with which the present invention is employed, is commonly known as a charge-injection device (CID). The principles underlying charge-injection device imagers are detailed in U.S. Pat. Nos. 3,805,062; 3,949,162; 4,000,418; 4,011,441 and 4,011,442, the disclosures of which are herein incorporated by reference.
In brief, a charge-injection device employs a silicon substrate having orthogonal row and column conductors thereon which are insulated both from the substrate and from each other. Each intersection of a row conductor with a column conductor provides two storage locations, one under the row conductor and the other under the column conductor, within which charges liberated from the silicon substrate by incident radiation may be stored by the application of appropriate voltages. The stored charges, when appropriately read out, form the video signal.
Using an appropriately doped silicon substrate such as, for example, an n-type semiconductor, a negative voltage applied to a row or column conductor is effective to produce a depletion region forming a potential well thereunder. The potential well functions as a capacitor to collect the charges liberated by incident radiation. Although mutually insulated, the potential wells under the row and column conductors at an intersection thereof are so closely coupled that charges may be transferred back and forth therebetween without loss of stored charge. Whichever one of the row and column conductors is maintained at the more negative potential captures all of the charge from the one maintained at a less negative potential. In order to transfer the charge from beneath one conductor to beneath the other conductor, the voltage on the conductor originally having the larger negative voltage is reduced to a value less than the negative voltage on the originally less negative conductor. Equivalently, the negative voltage on the previously less negative conductor may be increased until it exceeds the negative voltage on the first-mentioned conductor.
In one technique described in the referenced patents, at all times except during the reading-out process, the row conductors are maintained more negative than the column conductors. The liberated charges are therefore totally contained under the row conductors. In preparation for reading out a row, the row voltage is raised until it attains a less-negative voltage intermediate the column voltage and ground. This transfers all of the accumulated charges simultaneously in the selected row from beneath all of the row conductors to beneath their respective column conductors. The negative voltages on the column conductors are then increased one at a time in sequence to a less negative voltage than the selected row conductor. The less negative voltage may conveniently be zero volts. As the voltage on each column conductor is increased to zero, the charge stored thereunder flows back beneath its associated row conductor within the row being read out. The flow of charges in the row conductor occasioned by the transfer of charge from each column conductor is sensed to produce the output video signal. It should be noted that, since the only column conductors which contain charges are those in the selected row, the voltage sequence on the column conductors is ignored by all storage locations except those in the selected row.
The readout sequence described above is non-destructive; that is, at the end of reading the stored charges in a row, the charges, although they have been transferred first from beneath the row conductors to beneath the column conductors and then have been sequentially transferred back gain, remain in their original locations, undiminished. If the original voltages are restored on the row and column conductors, continued integration of incoming radiation without erasure of the previously stored charges may be performed. This is especially useful in low-light-level applications. In normal imaging applications, it is useful to erase the stored charges in a row just after it is read out so that a new charge pattern may be integrated until the next time the row is scheduled ior readout. The charges in a row are readily cancelled or erased by raising the selected row voltage to zero while the column voltages are also at zero. This injects sufficient charges into the storage locations to cancel any charge pattern which they may have acquired, and hence the name "charge-injection device".
Charge-injection devices have a high output impedence (several megohms and 12-15 pf) during the normal interval during which a horizontal line is displayed on the video monitor (line scan interval). Ideally, the load driven by the charge-injection device should have a low input impedence during the line scan interval, but a high input impedence during the charge-injection phase.
Noise is a problem in all imaging devices. The type of noise and its severity varies with the type of imaging device and with its required peripheral equipment. Charge-injection imaging devices suffer from two sources of noise giving rise to pattern noise; namely, switching noise and capacitance variation noise.
Reducing the input capacitance of the load driven by the charge-injection device lowers the overall noise of the imaging device. It has, however, heretofore been impractical to do so. Adding an element to the output of the charge-injection device to isolate the amplifier from the charge injection device injection device injection pulses increases input capacitance and resistance, degrading the signal-to-noise ratio thereof.
In the prior art, the charge-injection device drives a differential current-mode preamplifier as its load. This configuration has a high impedence during the charge-injection pulse, and a low impedence during the line scan interval, as is desired. However, this configuration prevents the use of sophisticated noise cancellation techniques because the only available output thereof corresponds to the difference in its two output currents. This severely limits the utility of the prior art imaging devices, as they are subject to the problems with noise noted above.